Advanced Entanglement from Quantum Dots

Communication is about to be revolutionized by the usage of quantum mechanics, known as the second quantum revolution. At the heart of this revolution are the exploitation of entangled states, which do not have any classical analogue. Therefore, the efficient creation of entangled states is crucial in this endeavor and the race of the search for the ideal source of entangled photon pairs is at its peak. The main goal of our project is to bring forward semiconductor quantum dots as usable source of entangled photon pairs. A new angle is added by a fundamentally new ingredient, namely the dark exciton states, for the controlled biexciton generation and subsequently the deterministic generation of time-bin entangled photons. We will consider advanced creation of time-bin entangled photon states, as well as multilevel entanglement and multiphoton entanglement using single quantum dots and quantum dot molecules. We aim to achieve these goals by a strong joint theoretical and experimental effort to turn the potential of quantum dots as entangled photon source into a useful set of tools for quantum communication, eventually leading to the construction of usable quantum repeaters.

Quantum physics opens up new possibilities compared to classical technologies and is thus revolutionizing our communication technology. The basis for this revolution are entangled states, which have no classical analogue. Therefore, the powerful generation of entangled states is essential for this effort and researchers worldwide are searching for ideal sources of entangled photon pairs. The aim of our project was to develop semiconductor quantum dots as a source for entangled photon pairs. A quantum dot is a semiconductor structure just a few nanometers in size that traps electrons and behaves like an atom. The main difference to conventional quantum light sources is that a quantum dot emits only one photon pair per excitation. This has already been shown for entanglement in the polarization (direction of oscillation) of photons, but polarization is unstable in in optical fibers. Our project therefore aimed to create entanglement in time. For this purpose, we needed so-called dark states of the quantum dots as new building blocks, which can be used for a controlled excitation of optically active states in the quantum dot and thus enable a deterministic generation of time-entangled photon pairs. To find and control these dark states, we developed completely new and groundbreaking methods to excite the quantum dots with laser light in close cooperation with the theory team in the project. These methods are based on specially structured laser pulses, which make it possible to achieve reliable excitation even with fluctuations in laser power and wavelength. Finally, we were also successful in exciting the dark states and we were able to bring a quantum dot into the dark state and back again in a controlled fashion. While we have only been able to show time entanglement to a limited extent so far, all doors are now open to us with the new excitation methods. In this way, we will take the functionality of quantum dots as sources of entangled photons to a higher level and take a significant step towards the realization of the quantum repeater, the heart of any future quantum communication network.